An Introduction to Ground Water Hydrology

Section 3 - More practical aspects of ground water hydrology

Aquifers

An aquifer is a geologic deposit that yields useful quantities of ground water to wells and springs. The term has meaning only in relation to the demands and expectations of the people using the aquifer; in fact, what we think of as an aquifer in Maine might be considered an aquiclude, a poorly permeable geologic deposit, in other parts of the country.

Both hydraulic conductivity and saturated thickness are important to the water-bearing characteristics of aquifers. Transmissivity of an aquifer is its hydraulic conductivity multiplied by its saturated thickness, and is a measure of the amount of water that can be transmitted horizontally by the full saturated thickness of the aquifer under a hydraulic gradient of 1. For comparison, good sand and gravel aquifers in Maine have transmissivities ranging from 50,000 to 100,000 gal/day/ft (length²/time, and good fractured bedrock aquifers have values ranging from 2,000 to 7,000 gal/day/ft.

Gravel Aquifers

Gravel aquifers differ greatly in water yield depending on the texture, permeability, saturated thickness, and recharge potential of the geologic formation. The highest yielding gravel aquifers in Maine are typically glacial eskers (locally called horsebacks or whalebacks) and deltas, glacial outwash deposits, and alluvial deposits associated with modern rivers. Household water supplies are available from a great variety of sand and gravel deposits. Aquifers suitable for municipal or industrial use typically have at least 10 feet of very permeable water-bearing sand and gravel, at least 20 feet of overall saturated thickness, and potential for recharge from precipitation and from a nearby surface water body. Under these favorable conditions, yields of 100 to over 1000 gallons per minute are obtainable from gravel aquifers.

Bedrock Aquifers

Bedrock aquifers occur wherever the crystalline rock is fractured, sufficiently saturated, and can be recharged by precipitation that percolates through overlying sediments. A typical household well is at least 100 feet deep in order to intersect sufficient saturated thickness and number of fractures. Variations in well yield are substantial from one place to another, ranging from essentially dry to very high yields of 300 to 500 gallons per minute. The difference in yield is primarily due to the intensity of fracturing, but also the the permeability and saturated thickness of overlying unconsolidated sediments from which recharge is derived. In Maine and northern New England, rock type generally does not significantly influence the yields of bedrock wells. Granite, for example, may be well fractured and water bearing in one area, and incapable of furnishing anything but a small domestic supply in another area. Important water-bearing fractures appear to be associated with faults. These features are detected through study of aerial photographs, geophysical exploration, and geologic evaluation of bedrock outcrops. Investigated in detail by test drilling and pumping, fractured bedrock aquifers are typically local zones of intensive fracturing that have definable strike, dip, and thickness over distances of a few hundred feet to as much as a mile. Significant bedrock aquifers in crystalline rocks are best thought of as high-yield zones within the bedrock.

Water Table and Artesian Aquifers

There are two principal kinds of aquifers: (1) those with a free surface at the water table and known as water-table aquifers and (2) those associated with confined ground water and known as artesian aquifers. The chief difference between the aquifers is the manner in which ground water is released from storage within them. This difference is important to the way in which water wells in these aquifers function and relates to the susceptibility of aquifers to contamination.

The water table has been described as the upper surface of the saturated zone, above which the interconnnected openings in the soil or rock are partly filled with air. If the water table falls, the depth of the overlying zone of aeration expands as air is drawn into the interconnected openings. If the water table rises, air is expelled into the atmosphere. Thus, the water table remains at atmospheric pressure at all times. At any depth below the water table, the hydrostatic pressure will be that caused by the weight of the overlying column of water plus the atmospheric pressure.

Ground water that is confined by a poorly permeable layer does not have free access to the atmosphere as does the water table. A confining layer must have at least 1/10 the permeablility of the underlying water-bearing layer. Recharge to a confined aquifer cannot cause a rise in water level, but causes a rise in water pressure. The pressure at the upper surface of a confined aquifer is greater than that of the atmosphere.

Artesian ground water conditions occur in both consolidated and unconsolidated materials. In Maine's glacial deposits, the most common situation is gravel overlain by clay as illustrated in Figure 15. The gravel is the aquifer and the clay and silt are the confining layer.

In bedrock the situation is somewhat different in that the aquifer is the fractured bedrock, and the confining layer is either relatively unfractured rock or overlying glacial clay and silt. Figure 16a is the most typical artesian system in crystalline bedrock in which solid rock confines water within a fracture zone. A more complex geologic situation is depicted in Figure 16b where the confining layer over the fractured bedrock aquifer is marine clay. Rain falling on the recharge area at some distance away and at higher elevation infiltrates the fractured bedrock and migrates down gradient to a discharge point at the flowing well. When the drill intersects this confined fracture at depth, the water in the well rises above the fracture, ocasionally overflowing at the ground surface.

Because the water from the recharge area is restricted in its upward flow, it exists under hydrostatic pressure. This pressure, or head, is indicated in Figure 16 by a dashed line sloping from the recharge area to the well. This line represents the potentiometric surface, which would be the natural elevation of the ground water if a well were drilled through the confining silt and clay layer and down into a fracture zone. The slope of the potentiometric surface is caused by frictional losses, called transmission head loss, as the water flows through the fractured bedrock aquifer. The slope of the potentiometric surface is the flow gradient of the artesian system, just as the water table is the flow gradient of a water-table system. Water will flow from points where this surface is high to points where it is low.

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